1. Field of the Invention
This present invention relates generally to electric motors, and more particularly, to induction motors.
2. Related Art
Electric motors are common place in modern society. An electric motor is an electric machine that converts electrical energy into mechanical energy. In normal motoring mode, most electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor. In certain applications, such as in the transportation industry with traction motors, electric motors can operate in both motoring and generating or braking modes to also produce electrical energy from mechanical energy. Electric motors may be powered by direct current (“DC”) sources, such as from batteries, motor vehicles or rectifiers, or by alternating current (“AC”) sources, such as from the power grid, inverters or generators.
An AC electric motor is an electric motor driven by an AC source that commonly includes two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field. Generally, there are two main types of AC electric motors, depending on the type of rotor used. The first type is the induction motor (also known as an asynchronous motor which may be a rotary induction motor or linear induction motor) and the second type is the synchronous motor. Induction motors are typically simple to construct, robust, capable of providing very high forces, and rely on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. In contrast, the synchronous motor does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency; however, synchronous motors tend to be relatively complex, as compared to induction motors, due to control logic that is typically needed to maintain synchronization of the synchronous motors. Other types of electric motors also include eddy current motors, and AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection; however, this disclosure is limited to induction motors.
Generally, an induction motor is an AC electric motor where the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding. The rotor of a rotary induction motor may be either wound type or squirrel-cage type. Generally, three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable, and economical. Turning to the linear induction motor, the linear induction motor operates on the same general principles as a rotary induction motor; however, a linear induction motor is typically designed to produce straight line motion. Uses of linear induction motors include magnetic levitation, linear propulsion, linear actuators, and liquid metal pumping. As an example, linear induction motors have been utilized for magnetic levitation (“maglev”) propulsion in public transportation systems such as high speed trains and other uses, such as, for example, direct X-Y motion for precision laser cutting, sliding doors, lifting mechanisms, etc.
Unfortunately, while induction motors are generally capable of producing high forces, under very high force conditions (such as, at maximum slip conditions), the operating current of an induction motor may be higher than the rated continuous current of the induction motor. This may lead to excessive heating and a limited duty cycle of the induction motor. As an example, under high slip conditions, an induction motor may operate at five times the normal rated continuous current which leads to excessive heating and a limited duty cycle that may be about 10 to 15 percent. Generally, known approaches to solve these problems include restricting the inertial loads and providing significant cooling to the linear induction motor. Unfortunately, this approach causes the induction motor to have slow acceleration, high cost, a requirement for precision mechanical assembly, difficulty in handling and manufacturing, loss of synchronization caused by control failures, and a significant cooling system. As such, there is a need to eliminate the large magnetized currents, improve efficiency, allow extended operation in maximum slip conditions, and allow high efficiency operation at very low speed and under high inertial loads.
An additional problem related to linear induction motors is remanence (also known as residual magnetic flux), which is a safety issue when power is lost to a linear induction motor during operation. As an example, if the linear induction motors are being utilized in the operation a maglev propulsion system (such as, for example a maglev train or heavy equipment), there is the possibility that a loss of power to the maglev propulsion system may case the train to lose magnetic levitation and potentially cause damage to the train and its content.
Approaches to solve this problem typically involve using permanent magnets in the induction motor to allow for the existence of a magnetic field even if there is a loss of power to the system. In a linear induction motor, this magnetic field would be capable of producing a breaking force against the linear motion of the maglev resulting in passive breaking that slows down the maglev and even passive holding (i.e., maintaining magnetic levitation) of the maglev.
Unfortunately, this approach requires the use of a large amount of permanent magnets. Permanent magnets are typically rare-earth magnets that are becoming scarcer and more expensive because of their increased applications in modern technology. In the case of a maglev train, the maglev propulsion system may include more than a hundred miles of track that may include a linear induction motor every 30 to 40 feet, where every linear induction motor includes numerous large rare-earth magnets. As a result, this system may include thousands of very expensive rare-earth magnets. Therefore, there is also a need for a system that utilizes less expensive magnetic materials that are capable of providing both passive breaking and passive holding of a linear induction motor.